Research conclusions: At present，according to the " Technical code for Railway Gas Tunnel" ，the absolute gas emission volume of 0.5 m3/ min is regarded as the boundary standard for grading the high gas and low gas．This standard is too low so that some tunnels that could be built with the standard of the low gas are regarded as the high gas tunnel．In this way，the construction investment goes to waste by taking some unnecessary measures，such as explosion －proof and reinforcing the tunnel lining．On the premise of meeting the ventilation requirement，it is more appropriate to regard the gas concentration of 0.5% in the return air flow as the the standard for grading the high gas and low gas．